Rain, sleet and snow are challenging enough for human drivers. Bad weather conditions present an extra level of difficulty for developers of self-driving cars that depend on cameras and sensors to “see” the road and make the correct decisions at 70 miles per hour.

Much work remains to get fully autonomous vehicles to operate under all conditions that can be managed by human drivers. But test vehicles deployed by automakers including General Motors Co., Ford Motor Co. and several automotive suppliers are part of a push to develop robotic cars and trucks that can handle black ice, snow, rain and other challenging weather conditions as well as — or better than — flesh-and-blood drivers.

Much of the first testing of self-driving cars was done in sunny Los Angeles, Arizona and Nevada. But the challenge of bad weather was no shock to those developing autonomous technology.

“We’ve been thinking about that right from the start,” said Jim McBride, Ford’s technical leader for autonomous vehicles. “We had to prioritize which problems come first.”

Ford tested self-driving Fusions in the snow last winter at Mcity, a 32-acre closed course in Ann Arbor. The automaker said then that the test vehicles’ 3-D maps helped the cars “see” when their suite of sensors, cameras, radar and lidar systems could not.

Autonomous vehicles use a series of redundant systems to navigate roadways. Developers vary on which sensor, camera or mapping program is the dominant driver of the vehicle, but each of the systems supplement the others. In inclement weather, one of those systems could be negatively impacted — a camera could be caked in ice, for example.

McBride said the redundancies are key: If the car can detect ice-slicked roads through its sensor suite, or through vehicle-to-vehicle or vehicle-to-infrastructure communication, it can activate the proper electronic systems that are already on vehicles to react.

Reaction, in some ways, is the simple part. Autonomous technology developers know how to make a car drive itself. It’s making the car see through visual noise such as snow or buildup on the sensor that is challenging.

Amine Taleb, research and development director at automotive supplier Valeo, is working on the “seeing” side of the equation. Taleb and his team are working to enhance how well the sensors can see, and at the same time working out ways to deal with glare from the sun and slush, ice or condensation. Some sensors can be heated to melt frozen slush; others can can be washed, blown dry or wiped clean like a windshield.

“We will face inclement weather,” Taleb said. “We cannot avoid it. In high-level automation, you do need basically multiple redundancies, no matter what the condition on the road is. As we gain more knowledge and optimization of autonomous driving and these different conditions, we start to see more inclement-weather driving.”

The first autonomous vehicles expected on the road in the early 2020s will likely be “geo-fenced” and confined to specific well-mapped areas for ride-hailing, ride-sharing or delivery services. In those scenarios, automakers will have more control over how and when the vehicles are used.

“To some extent, they will be largely fair-weather vehicles at first, although that doesn’t mean sunshine only,” said Sam Abuelsamid, analyst with Navigant Research, which tracks emerging technologies such as autonomous driving. “In fleet use they will return to base daily and maintenance crews will make sure washer-fluid tanks get topped up daily, something a regular use probably wouldn’t do.

“Maybe someday they will handle bad weather better than a good human driver, but it will likely take a while to get there,” he said. “They will probably handle the conditions better than many human drivers, though.”

The earliest models sold to consumers will probably require the driver to take over in inclement weather, he said. Some cars already do this if a sensor gets covered with road salt.

The goal for most companies, though, is to make a car that can drive itself in any weather a human would. That could be a boon for Michigan and other northern states that early on in autonomous vehicle development were snubbed by the companies testing those machines.

The American Center for Mobility being built at Willow Run airport in Ypsilanti will open the first phase of a multi-phase plan in December. Automakers, suppliers and universities will be able to rent time at the center, which will eventually have a long tunnel in which fog, sleet, snow, rain and other weather conditions can be simulated

“Our weather is actually a positive for autonomous testing,” Gov. Rick Snyder said at a mid-October event at the center. “You need all four seasons, and a lot of these places don’t have it. It’s great that we’ll have the best environment because of having the four seasons.”

This winter, the center will be testing the vehicles on icy roads, John Maddox, president of the American Center for Mobility, said.

Navigant’s Abuelsamid said capability in all weather conditions will be mastered eventually. “In some conditions, functionality may be limited,” he said. “But it is with human drivers as well.”